The present disclosure relates to a technology of a developing roller that is disposed to face, without contact, the outer circumferential surface of the image carrier.
A developing device is installed in an image forming apparatus which is a copier, a printer or the like and forms an image on a paper sheet based on the electrophotography. The developing device develops, by toner, an electrostatic latent image formed on an image carrier such as a photoconductor drum. As the developing method, a so-called two-component developing method is known which uses two-component developer including magnetic carrier and toner to develop a toner image on the image carrier. As an example of the two-component developing method, there is conventionally known a contactless developing system called “interactive touchdown developing system”. In the interactive touchdown developing system, a developing roller and a magnetic roller are used. The developing roller is disposed at a predetermined distance from the image carrier. A magnet is embedded in the magnetic roller. The magnetic roller draws up the magnetic carrier as well as the toner, and holds them on the surface thereof. The magnetic roller forms a magnetic brush thereon to transfer only the toner to the developing roller, and form a toner thin layer on the developing roller. An AC electric field is generated by a developing bias that includes an AC component applied to the developing roller, and the AC electric field flies the toner from the developing roller and causes the toner to adhere to the electrostatic latent image on the image carrier.
There is known a developing roller used in this type of developing device, wherein in the developing roller, a base body made of aluminum is coated with an alumite layer, and the alumite layer is coated with a resin coat layer that is electrically conductive. The electrical conductivity of the resin coat layer is realized by mixing a conductive agent, such as titanium oxide, with a resin material that is highly insulating. It is noted that the alumite layer plays a role in suppressing a leak from occuring between the image carrier and the developing roller.